LOWER HELDERBERG ROCKS. 
169 
Fig. 1 a, b, c. Views of small individuals of this species, from Maryland. 
Fig. 2 a , b, c, & 3. Larger individuals, from New-York, showing the ventral and dorsal 
sides, profile and area. 
Fig. 4. The interior of the dorsal valve of the smaller individuals. 
Fig. 5. Interior of the ventral valve. 
Fig. 6 a, b. Dorsal and ventral valves of a cast of the same species. 
Geological position and locality. In the shaly limestone of the Lower Helderberg 
group : Helderberg mountains, Albany county; and in the same position, and also 
in the Oriskany sandstone, Cumberland, Maryland. 
Ortliis subcarinata (n. s ). 
Plate XII. Fig. 7 - 21. 
Shell somewhat transversely oval, often nearly circular or quadrangular, 
plano-convex. Dorsal valve more or less flattened, with a distinct 
depression along the middle, which becomes wider towards the base, 
producing a sinus in the margin of the shell : beak scarcely rising 
above the hinge line. Ventral valve very convex, strongly elevated or 
subcarinate along the middle : beak small, incurved. Area narrow, 
linear, one-half to two-thirds the width of the shell. The depressed 
line along the middle of the dorsal valve, and the ridge upon the ven¬ 
tral valve, very frequently diverge from the central line, though the 
specimens do not appear to have suffered any distortion from pressure. 
Surface finely striated : striae curving upwards towards the lateral and 
cardinal margins, equal or alternating in size, and not unfrequently 
fasciculate; concentrically marked by strong lines of growth towards 
the margin, and, in well-preserved specimens, by fine striae over the 
entire surface. 
Interior of the dorsal valve with an obscurely double impression for 
the adductor muscles on each side of a low sharp elevation, with sides 
subparallel : cardinal process strong and bifid at the extremity, with 
each division bilobed; brachial processes strong, divergent; vascular 
impressions preserved in the cast. The ventral valve shows, beneath 
the beak, a strong triangular cavity, with an abrupt groove at each 
[ Palaeontology III.] 22 
